A wide variety of metal-oxide semiconductor field-effect transistors (MOSFETs) are utilized in motor vehicles to drive inductive loads, such as solenoids. In a typical application, a control signal, provided by a gate drive circuit, is applied across a gate and a source of a MOSFET to control energization of a solenoid that is coupled to a drain of the MOSFET. In a typical application, the control signal periodically causes the MOSFET to turn off. In this case, the gate drive circuit typically attempts to pull the MOSFET gate low to reduce the gate voltage. As the gate voltage is reduced, the MOSFET starts to turn off and the current through the MOSFET attempts to go down. However, since the inductive load coupled to the drain of the MOSFET requires a fixed current to flow for a short period of time, the drain voltage of the MOSFET rises such that the MOSFET conducts substantially the same level of current at the lower gate voltage. In this situation, if the MOSFET is not protected, the voltage across the drain and source of the MOSFET can rise to a point where the MOSFET undergoes breakdown.
To prevent the MOSFET from undergoing breakdown, a stack of Zener diodes have traditionally been connected between the gate and drain of the MOSFET (see FIG. 6, where the stack is represented by one Zener diode). When the drain-to-gate voltage reaches the breakdown voltage of the stack, the Zener diodes breakdown passing current into the gate, which, in turn, holds the gate voltage at some minimum acceptable level to allow current to flow through the inductive load. Unfortunately, in this case, the current through and the voltage across the MOSFET can reach relatively high values, resulting in high power dissipation by the MOSFET, which, in turn, can increase a temperature of the MOSFET. Thus, in a typical power MOSFET, in order for the MOSFET temperature to be kept below a critical value, the size of the MOSFET may be required to be relatively large. In the case where the device is physically larger than required to reach a specified maximum on-resistance, the device is said to be thermally limited.
What is needed is a technique for protecting a semiconductor device from energy transients that generally does not require the physical size of the device to be larger than that required to reach a specified maximum on-resistance.